In order for Anger gamma cameras to form an image showing the distribution of radioactive material in an object or in a patient, a means is necessary to determine the location of the radioactive material. This means usually comprises of a collimator attached to the face of the camera to control the direction of the detected gamma rays or other radiation emanating from the radioactive material. The control of directionality occurs at each location on the camera face by means of collimation channels which allow gamma rays (or other radiation) through only if they come from within an acceptance angle.
In a parallel-hole collimator, the apertures are parallel to each other, perpendicular to the crystal of the camera face, long enough and of small enough diameter that the acceptance angle is narrow. The apertures are packed closely enough together, in most cases, that the intrinsic resolution of the camera does not allow resolution of the apertures on the final image. The result is an acceptable 1:1 relation between direction of origin of the gamma rays and site of interaction with the camera crystal. This allows an image to be formed by film or a computer since the electronics of the camera are able to localize the site of interaction of each gamma ray with the crystal.
Gamma cameras sometimes are used in connection with so called high-energy isotopes. Many references define “high energy” or “super high energy” isotopes as those that undergo positron emission and the accompanying 511 keV photons (e.g., fluorine-18, iodine-123, carbon-11, nitrogen-13 and oxygen-15). As such, these collimators are designed around the 511 keV emission, and generally have a working range not in excess of 600 keV. Sodium-24 is a radioactive isotope produced by the neutron irradiation of stable sodium (Na-23). Sodium is found in many products, with specific mention to pharmaceutical dosage forms such as capsules and tablets. As such, sodium-24 is a desirable isotope for radionuclide imaging studies. However, sodium-24 has gamma rays of 1368 and 2754 KeV—well beyond the range of many of the so-called “super high energy” known collimators. Particularly, the gamma rays from sodium-24 penetrate through known collimator septa resulting in a scatter of gamma photons, which interact with the crystal, or detector, of the gamma camera resulting in reduced image resolution.
Accordingly, there is a need for a collimator that reduces the scatter of high-energy gamma photons, e.g. those from sodium-24, thereby providing enough resolution for high energy gamma photon, e.g. sodium-24, radionuclide imaging using a gamma scintillation camera.